1. Field of the Invention
This invention relates to a circuit for stabilizing the output of an injection laser being used as the light source in a digital lightwave communications system.
2. Description of the Prior Art
It is known that the operating characteristics of most injection lasers vary significantly with aging and with ambient temperature. In particular, typically the lasing threshold increases with temperature and age while the slope of the light-current transfer characteristic decreases with temperature and age. Additionally, with time some lasers develop nonlinearities in the light-vs-current transfer characteristic.
In digital lightwave systems, particularly those operating at high bit rates, it is important to compensate for these changes if the operation is to be relatively maintenance free for extended periods of time.
In such systems, lasers are usually biased with a d-c current I.sub.b near the lasing threshold and a modulation current I.sub.m is superimposed on the bias to swing the light output from its "OFF" state value L.sub.o, selectively under signal control, to its "CN" state value L.sub.1. Changes in L.sub.o and/or L.sub.1 degrade the performance of the system and so are undesirable.
To achieve the desired compensation with aging of the critical parameters, it is common to vary appropriately these two currents so that any two linear combinations of the "OFF" light lever L.sub.c, "ON" light level L.sub.1 and the time-averaged light value L.sub.av are kept constant. Since L.sub.av is proportional to the duty cycle of the data signal, a measure of the duty cycle has to be provided to the control arrangements if the duty cycle is not constant. The duty cycle is the fraction of the time that the laser is in its "ON" state. Several control arrangements based on these currents have been proposed hitherto. However, these all suffer from one or more deficiencies.
In particular, at the Second European Conference on Optical Fiber Communications, held in Paris, France, in 1976, there was presented a paper entitled "Subsystems for High Speed Optical Links" which described a control arrangement in which the "OFF" level L.sub.o was kept constant by feedback control of the bias I.sub.b, while the difference L.sub.1 -L.sub.o was kept constant by feedback control of the modulating current I.sub.m. It is characteristic of control arrangements, which depend on control of L.sub.o, that they involve the difference of two relatively large values, each approximately equal to the average light value L.sub.av. As is well known, such arrangements generally are undesirable because a relatively small error in deriving either of the two large values results in a disproportionately large error in their difference. Moreover, this arrangement depends on an idealistic assumption that the duty cycle does not vary with time. However, such assumptions are not always warranted in practice.
There has appeared in the IEEE Transactions on Communications COM-26(7) 1088, (1978) a paper entitled "Electronic Circuits for High Bit Rate Digital Fiber Optic Communications Systems." In this paper, there is recognized the importance of the duty cycle and there is proposed a control arrangement which is relatively independent of duty cycle. However, this arrangement also is based on using the bias current to control L.sub.o, with the same objection that this involves deriving the difference of two comparably large quantities with the resultant disadvantage mentioned.
In the Fourth European Conference on Optical Fiber Communications, held at Genoa, Italy, in 1978, there was presented a paper entitled "Level Control Circuit for Injection Laser Transmitters." This paper describes a control arrangement in which the bias current I.sub.b is used to keep constant the average light L.sub.av, and the modulating current I.sub.m is used to keep constant the difference L.sub.1 -L.sub.o. However, this arrangement too is dependent on the duty cycle and so is vulnerable to changes in it.
In Electronics Letters, 23 Nov. 1978, Vol. 14, No. 24, there appears a paper entitled "Laser Level Control Circuit for High-Bit-Rate Systems Using a Slope Detector." This paper describes a circuit that detects the change of slope of the light-output L/Current I transfer characteristic of the laser at the threshold knee to control the difference L.sub.1 -L.sub.o, and a mean-power circuit is used to keep constant the average light L.sub.av. This circuit is vulnerable to nonlinearities in the transfer characteristic away from the threshold knee of the characteristic, and this arrangement too is dependent on the duty cycle.
An object of my invention is a control circuit which essentially avoids all of the various difficulties mentioned for the arrangements discussed above.